Lead dioxide/fluoride-containing cathodes for solid electrolyte cells

ABSTRACT

A cathode for use in solid electrolyte cells comprising a mixture of water-containing lead dioxide and a metal fluoride.

FIELD OF THE INVENTION

The invention relates to a cathode for use in solid state electrolyte cell systems in which the cathode comprises a mixture of lead dioxide with a metal fluoride.

BACKGROUND OF THE INVENTION

Ionic conductivity is usually associated with the flow of ions through an aqueous solution of metallic salts. In the vast majority of practical uses of ionic conductors, e.g., as electrolytes for dry cell batteries, the aqueous solution is immobilized in a paste or gelled matrix to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the necessity of including a large volume of immobilizing material has hindered the aims of miniaturization.

In attempting to overcome the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds hoping to find compounds which are solid at room temperature and have ionic conductances approaching those exhibited by the commonly used liquid systems. Such compounds have specific conductances at room temperature (20° C.) in the range of 10⁻⁶ to 10⁻¹⁵ ohm⁻¹ cm.⁻¹ as compared to aqueous solutions of salts which typically have a specific conductance of 0.5 to 0.05 ohm⁻¹ cm⁻¹.

Improved microelectronic circuit designs have generally decreased the current requirements for electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can only deliver currents in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase and corrosion phenomena. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.

Solid electrolytes must be essentially electronic insulators so as not to internally short the cell while at the same time they must allow ionic movement in the crystal lattice for the cell to operate. It has been discovered that certain metallic salts which are solids at room temperatures have ionic conductances sufficiently high to permit their use in practical battery applications. For example, U.S. Pat. No. 3,723,185 discloses solid state electrolytes of compounds conforming to the general formula AgI-MCN-AgCN or modifications thereof wherein M is potassium, rubidium, cesium or mixtures thereof.

U.S. application Ser. No. 973,554 filed Dec. 27, 1978 titled Solid State Electrolyte, discloses a relatively highly conducting solid state electrolyte made from a binary mixture of lead fluoride and a 0.1 to 15.0 mole percent of a selected alkali salt of sodium, potassium, rubidium and cesium. U.S. application Ser. No. 973,552 filed Dec. 27, 1978 titled Solid State Electrolytes discloses a relatively highly conducting solid state electrolyte of a product of a fused ternary mixture of lead fluoride, an alkali salt of sodium, potassium, rubidium or cesium in a range of 0.1 to 15.0 mole percent and a fluoride, nitrate or sulfate of magnesium, calcium, strontium or barium in a range of between 1 and 40 mole percent. Both of these U.S. applications are incorporated herein by reference.

Although various solid cathode materials are recited in the art for use in various cell systems, an object of the present invention is to provide a novel cathode for use in solid electrolyte cell systems employing solid electrolytes such as those disclosed in the above-identified U.S. patent applications Ser. Nos. 973,552 and 973,554.

Another object of the present invention is to provide a solid cathode for use with a solid fluoride-containing electrolyte wherein said cathode comprises a mixture of lead dioxide and a metal fluoride.

Another object of the present invention is to provide a cathode consisting essentially of lead dioxide and a metal fluoride for use in a lead fluoride-containing solid electrolyte system, and wherein the open circuit voltage of the system is higher than the open circuit voltage obtained using either lead dioxide or a metal fluoride alone as the cathode in the system.

Another object of the present invention is to provide a cathode consisting of lead dioxide and a metal fluoride for use in a solid electrolyte cell employing a lead anode and a lead fluoride-based solid electrolyte as disclosed in U.S. applications Ser. Nos. 973,552 and 973,554.

Another object of the present invention is to provide a cathode consisting of a mixture of lead dioxide and a metal fluoride in a mole ratio of between about 3:1 and about 1:3.

The foregoing and additional objects will become more fully apparent from the following description.

SUMMARY OF THE INVENTION

The invention relates to a cathode for use in an electrochemical cell comprising a mixture of lead dioxide (PbO₂) having a water content of between about 0.4 and about 10 weight percent with at least one metal fluoride in a mole ratio of between about 3:1 and about 1:3, preferably about 1:1. The metal fluoride can be selected from the group consisting of cobaltic trifluoride (CoF₃), lead trifluoride (PbF₃), manganese trifluoride (MnF₃), tantalum pentafluoride (TaF₅), niobium pentafluoride (NbF₅), mercurio fluoride (HgF₂), mercurous fluoride (Hg₂ F₂), cupric fluoride (CuF₂), argentous fluoride (AgF), argentic fluoride (AgF₂) and bismuth trifluoride (BiF₃). The preferred metal fluorides are cobaltic trifluoride, manganese trifluoride, argentic fluoride and mercuric fluoride, with the most preferable being cobaltic trifluoride and manganese trifluoride.

It was unexpectedly found that when lead dioxide was mixed with a metal fluoride such as manganese trifluoride, cobaltic trifluoride or tantalum pentafluoride, and then used as a cathode in a cell employing a lead anode, a solid electrolyte of 88PbF₂ -8SrF₂ -4KF, the open circuit voltage was higher than the open circuit voltage using either lead dioxide or the metal fluoride as the cathode in the cell system. Although not wanting to be bound by theory, a possible explanation of the synergistic effect of using lead dioxide together with the metal fluoride is that the water in the lead dioxide (PbO₂) reacts with the metal fluoride such as manganese trifluoride to produce hydrogen fluoride (HF) as follows:

    2MnF.sub.3 +3H.sub.2 O→6HF+Mn.sub.2 O.sub.3.

Hydrogen fluoride is then believed to take part in the overall cell reaction as follows: ##EQU1##

It has been reported in the art that lead dioxide always contains Pb²⁺ and OH⁻ ions as replacement for some Pb⁴⁺ and O²⁻ ions. Chemical analysis of alpha-PbO₂ (orthorhombic-columbite structure) and beta-PbO₂ (rutile) have been reported by N. E. Bagshaw, R. L. Clarke and B. Halliwell, in the Journal of Applied Chemistry, 16, P. 180 (1966) as follows:

                  TABLE I                                                          ______________________________________                                         Chemical analysis of Alpha-lead dioxide                                                    Lead,   Oxygen   Hydro-                                                                               Chemical                                    Preparation method                                                                         %       %        gen % Composition                                 ______________________________________                                         Sodium chlorate-                                                               sodium                                                                         nitrate oxidation                                                                          86.60   13.17    0.057 PbO.sub.1.83 (OH).sub.0.14                  Chlorine dioxide                                                               oxidation   85.84   13.69    0.106 PbO.sub.1.81 (OH).sub.0.26                  Ammonium per-                                                                              86.19   13.70    0.095 PbO.sub.1.83 (OH).sub.0.23                  sulphate oxidation                                                             Alkaline electro-                                                              formation   85.89   13.05    0.068 PbO.sub.1.80 (OH).sub.0.16                  Alkaline-lead                                                                  acetate                                                                        electrodeposition                                                                          85.95   13.42    0.082 PbO.sub.1.82 (OH).sub.0.19                  ______________________________________                                    

                  TABLE II                                                         ______________________________________                                         Chemical analysis of Beta-lead dioxide                                                     Lead,   Oxygen   Hydro-                                                                               Chemical                                    Preparation method                                                                         %       %        gen % Composition                                 ______________________________________                                         Lead tetra-acetate                                                             hydrolysis  85.85   13.64    0.083 PbO.sub.1.86 (OH).sub.0.20                  Nitric acid oxidation                                                          of minium   85.54   13.35    0.045 PbO.sub.1.91 (OH).sub.0.11                  Acid-       85.69   13.67    0.106 PbO.sub.1.81 (OH).sub.0.26                  electroformation                                                               Acid-lead                                                                      perchlorate                                                                    electrodeposition                                                                          86.19   13.91    0.044 PbO.sub.1.98 (OH).sub.0.11                  Acid-lead acetate                                                              electrodeposition                                                                          85.60   13.85    0.073 PbO.sub.1.92 (OH).sub.0.18                  ______________________________________                                    

It has been observed that extreme drying of PbO₂ by long containment in a dry box of about 1 ppm H₂ O vapor apparently leads to sufficient dehydration to substantially interefere with the cell reaction, and then when mixtures of MnF₃ and this dry "PbO₂ " are used as cathode materials, open circuit voltages of vanishingly small magnitudes are obtained that only slowly rise to 1.75 to 1.90 V. The attainment of the expected open circuit voltage value is accelerated by slight heating or by the addition of small amounts of water to the PbO₂ prior to mixing with MnF₃. It has been found that from about 0.4% by weight to as much as about 10% by weight water can be added and preferably between about 3% to about 6%. Lead dioxide having a water content of about 0.4 percent would have the following formula:

    PbO.sub.1.79 (OH).sub.0.2.0.06H.sub.2 O

The cathode of this invention could be used with fluoride ion-conducting solid electrolytes, preferably PbF₂ -based solid electrolytes. Examples of solid electrolytes for use in conjunction with the cathodes of this invention are as follows:

    ______________________________________                                         Mixture             Specific Resistivity                                       Composition (Molar Proportions)                                                                    (ohm-cm) at Room Temp.                                     ______________________________________                                         86PbF.sub.2 -10CaF.sub.2 -4KF                                                                      1578                                                       81PbF.sub.2 -15CaF.sub.2 -4KF                                                                      2750                                                       76PbF.sub.2 -20CaF.sub.2 -4KF                                                                      2200                                                       71PbF.sub.2 -25CaF.sub.2 -4KF                                                                      2500                                                       66PbF.sub.2 -35CaF.sub.2 -4KF                                                                      1730                                                       61PbF.sub.2 -35CaF.sub.2 -4KF                                                                      1800                                                       79PbF.sub.2 -6KF-15SrF.sub.2                                                                       10778                                                      77PbF.sub.2 -8KF-15SrF.sub.2                                                                       1736                                                       75PbF.sub.2 -10KF-15SrF.sub.2                                                                      3000                                                       86PbF.sub.2 -10BaF.sub.2 -4KF                                                                      1100                                                       81PbF.sub.2 -15BaF.sub.2 -4KF                                                                      2800                                                       76PbF.sub.2 -20BaF.sub.2 -4KF                                                                      4400                                                       71PbF.sub.2 -25BaF.sub.2 -4KF                                                                      14000                                                      66PbF.sub.2 -30BaF.sub.2 -4KF                                                                      113000                                                     61PbF.sub.2 -35BaF.sub.2 -4KF                                                                      132000                                                     56PbF.sub.2 -40BaF.sub.2 -4KF                                                                      483000                                                     86PbF.sub.2 -10SrF.sub.2 -4RbF                                                                     2660                                                       86PbF.sub.2 -10SrF.sub.2 -4KF                                                                      1477                                                       81PbF.sub.2 -15SrF.sub.2 -4KF                                                                      2413                                                       76PbF.sub.2 -20SrF.sub.2 -4KF                                                                      3544                                                       71PbF.sub.2 -25SrF.sub.2 -4KF                                                                      9310                                                       66PbF.sub.2 -30SrF.sub.2 -4KF                                                                      26000                                                      61PbF.sub.2 -35SrF.sub.2 -4KF                                                                      70000                                                      56PbF.sub.2 -40SrF.sub.2 -4KF                                                                      269000                                                     88PbF.sub.2 -8SrF.sub.2 -4LiF                                                                      380000                                                     88PbF.sub.2 -8SrF.sub.2 -4RbCl                                                                     4100                                                       76PbF.sub.2 -20CaF.sub.2 -4RbF                                                                     5200                                                       76PbF.sub.2 -20BaF.sub.2 -4RbF                                                                     6500                                                       81PbF.sub.2 -15SrF.sub.2 -4KBr                                                                     5200                                                       81PbF.sub.2 -15SrF.sub.2 -4K.sub.2 CO.sub.3                                                        7100                                                       81PbF.sub.2 -15SrF.sub. 2 -4K.sub.2 SO.sub.4                                                       8500                                                       76PbF.sub.2 -20SrF.sub.2 -4KBr                                                                     7500                                                       76PbF.sub.2 -20SrF.sub.2 -4K.sub.2 CO.sub.3                                                        9800                                                       76PbF.sub.2 -20SrF.sub.2 -4K.sub.2 SO.sub.4                                                        18400                                                      96PbF.sub.2 -2CaF.sub.2 -2KF                                                                       1326                                                       94PbF.sub.2 -3CaF.sub.2 -3KF                                                                       827                                                        95PbF.sub.2 -2CaF.sub.2 -3KF                                                                       766                                                        94PbF.sub.2 -2CaF.sub.2 -4KF                                                                       642                                                        95PbF.sub.2 -3CaF.sub.2 -2KF                                                                       1134                                                       94PbF.sub.2 -1CaF.sub.2 -5RbCl                                                                     3450                                                       96PbF.sub.2 -2MgF.sub.2 -2KF                                                                       3630                                                       94PbF.sub.2 -1MgF.sub.2 -5KF                                                                       2430                                                       94PbF.sub.2 -2MgF.sub.2 -4KF                                                                       1108                                                       96PbF.sub.2 -2MgF.sub.2 -2K.sub.2 CO.sub.3                                                         4400                                                       94PbF.sub.2 -1MgF.sub.2 -5K.sub.2 CO.sub.3                                                         1047                                                       94PbF.sub.2 -1MgF.sub.2 -4K.sub.2 CO.sub.3                                                         1161                                                       88PbF.sub.2 -8MgF.sub.2 -4KF                                                                       26400                                                      86PbF.sub.2 -10MgF.sub.2 -4KF                                                                      21900                                                      76PbF.sub.2 -20MgF.sub.2 -4KF                                                                      40200                                                      88PbF.sub.2 -8SrF.sub.2 -4NaF                                                                      20000                                                      88PbF.sub.2 -8SrF.sub.2 -4RbF                                                                      5700                                                       88PbF.sub.2 -8SrF.sub.2 -4CsF                                                                      63000                                                      90PbF.sub.2 -5SrF.sub.2 -5KF                                                                       848                                                        88PbF.sub.2 -8SrF.sub.2 -2K.sub.2 CO.sub.3                                                         1970                                                       90PbF.sub.2 -8SrF.sub.2 -2K.sub.2 CO.sub.3                                                         1202                                                       86PbF.sub.2 -10CaF.sub.2 -2K.sub.2 CO.sub.3                                                        1815                                                       86PbF.sub.2 -10SrF.sub.2 -2K.sub.2 CO.sub.3                                                        1405                                                       96PbF.sub.2 -2Sr(NO.sub.3).sub.2 -2KNO.sub.3                                                       1078                                                       86PbF.sub.2 -10Sr(NO.sub.3).sub.2 -4KNO.sub.3                                                      8290                                                       86PbF.sub.2 -10SrF.sub.2 -4KNO.sub.3                                                               1450                                                       *88PbF.sub.2 -8SrF.sub.2 -4KF                                                                      857                                                        90PbF.sub.2 -6SrF.sub.2 -4KF                                                                       1034                                                       94.5PbF.sub.2 -0.5CaSO.sub.4 -5.0KF                                                                1136                                                       94.5PbF.sub.2 -0.5CaSO.sub.4 -5.0K.sub.2 CO.sub.3                                                  1164                                                       88PbF.sub.2 -8SrF.sub.2 -4K.sub.2 SO.sub.4                                                         5700                                                       88PbF.sub.2 -8SrF.sub.2 -4KNO.sub.3                                                                1900                                                       88PbF.sub.2 -8SrF.sub.2 -4KCI                                                                      20000                                                      88PbF.sub.2 -8SrF.sub.2 -4KBr                                                                      2500                                                       88PbF.sub.2 -8SrF.sub.2 -4K.sub.2 CO.sub.3                                                         1700                                                       88PbF.sub.2 -8BaF.sub.2 -4KF                                                                       1050                                                       88PbF.sub.2 -8BaF.sub.2 -4K.sub.2 SO.sub.4                                                         5900                                                       88PbF.sub.2 -4CaF.sub.2 -4SrF.sub.2 -4KF                                                           4000                                                       88PbF.sub.2 -4CaF.sub.2 -4SrF.sub.2 -4K.sub.2 SO.sub.4                                             15600                                                      50PbF.sub.2 -50SnF.sub.2                                                                           1000                                                       ______________________________________                                          *The ionically conducting product formed from this ternary mixture was         analyzed and found to contain the following: 78.63 wt. percent lead, 3.07      wt. percent strontium, 15.97 wt. percent fluorine and 0.60 wt. percent         potassium.                                                               

Additional solid state electrolytes for use in conjunction with the cathode of this invention would include the ionically conductive product of a binary mixture of lead fluoride (PbF₂) and a range of between about 0.1 and about 15.0 mole percent of cesium fluoride or a metal salt having a cation component selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs) and an anion component selected from the group consisting of chloride, bromide, iodide, cyanide, nitrate, hydroxide, thiocyanate, carbonate, phosphate, sulfate, oxide, formate, acetate and oxalate.

The preferred solid electrolytes for use in conjunction with the cathodes of this invention are 88PbF₂ -8SrF₂ -4KF, 86PbF₂ -10BaF₂ -4KF, 94PbF₂ -3CaF₂ -3KF, 95PbF₂ -2CaF₂ -3KF, 95PbF₂ -5KF and 50PbF₂ -50SnF₂.

Suitable anodes for use in conjunction with the cathode of this invention are lead, lead-coated substrates, alloys of lead and metals more nobel than lead.

DRAWINGS

FIGS. 1, 2 and 3 are graphs illustrating the discharge characteristics of different cells made in accordance with this invention and as described in Examples I, II, and IV, respectively.

EXAMPLE 1

0.232 gram of cobalt trifluoride (CoF₃) was mixed with 0.245 gram lead dioxide (PbO₂) and then 0.150 gram of the solid electrolyte 88PbF₂ -8SrF₂ -4KF was added. The cathode so formed was used with a solid electrolyte 88PbF₂ -8SrF₂ -4KF and a lead disc (anode) to produce a solid electrolyte cell identified as cell A. Two additional cells of the same type were produced except that in the first of these cells, identified as cell B, only 0.116 gram of cobalt trifluoride was used and in the second cell, identified as cell C, 0.348 gram of cobalt trifluoride was used. The three cells were discharged across a 10-kilohm load and the data obtained are shown in FIG. 1. As evident from these data, the cathode of this invention can be used as a cathodic component for solid electrolyte cells.

EXAMPLE II

0.112 gram of manganese trifluoride (MnF₃) was mixed with 0.245 gram of lead dioxide and then 0.357 gram of the solid electrolyte 88PbF₂ -8SrF₂ -4KF was added. The cathode so formed was used with a solid electrolyte of 88PbF₂ -8SrF₂ -4KF and a lead disc (anode) to produce a solid electrolyte cell. The cell was discharged across a 300-kilohm load and the data obtained are shown plotted in the voltage vs. time curved in FIG. 2.

EXAMPLE III

Table 2 presents data showing various cathodes made in accordance with this invention and employed in cells along with a lead disc anode and a solid electrolyte 88PbF₂ -8SrF₂ -4KF. Each of the cells so produced was discharged across a 100-kilohm resistor and the closed circuit voltage was observed. These data along with the open circuit voltage of each cell are shown in Table 2.

                  Table 2                                                          ______________________________________                                                      Open Circuit Voltage                                                                           Closed Circuit                                    Cathode      (volts)         Voltage (volts)                                   ______________________________________                                         AgF.sub.2 --PbO.sub.2 *                                                        (1:1M)       1.92            1.87                                              TaF.sub.5 --PbO.sub.2 *                                                                     1.91            1.85                                              (1:1M)                                                                         MnF.sub.3 --PbO.sub.2 *                                                                     1.85            1.65                                              (1:1M)                                                                         CoF.sub.3 --PbO.sub.2 *                                                                     1.95            1.88                                              (1:1M)                                                                         M = mole ratio                                                                 ______________________________________                                          *PbO.sub.2 containing 0.4% H.sub.2 O.                                    

EXAMPLE IV

Seven groups of cells containing various amounts of water were made. Each group consisted of three identical cells. Each cell contained a lead anode and an electrolyte layer composed of 88 mole % PbF₂, 8 mole % SrF₂, 4 mole % KF. The cathode of each cell contained 24.52% MnF₃, 23.08% of the above solid electrolyte, 2.00% Teflon powder binder and a quantity of PbO₂ and water which totaled 50.40%. The PbO₂ and H₂ O percentages are shown in Table 3. The PbO₂ had been exposed to a dry box atmosphere of less than one ppm water and had lost some of its water.

The average open circuit voltage of the three cells in each group is shown in Table 3. As can be seen, it is proportional to the amount of water contained in the cathode. Each cell was then heated for 20 minutes at 75° C., stored at room temperature for 20 hours, and again the average open circuit voltage of the three cells in each group was measured and is shown in Table 3.

                  Table 3                                                          ______________________________________                                         Percent of H.sub.2 O Added                                                                       Average Open Circuit Voltage                                 Group PbO.sub.2   H.sub.2 O                                                                              Initial   After Heating                              ______________________________________                                         A     50.40       0       0.04      0.20                                       B     50.30       0.10    0.04      0.22                                       C     50.10       0.30    1.14      1.45                                       D     49.40       1.00    1.45      1.73                                       E     47.40       3.00    1.59      1.81                                       F     44.40       6.00    1.65      1.93                                       G     40.40       10.00   1.78      0.35                                       ______________________________________                                    

Two cells from Group E were discharged across a 100-kilohm resistor and the data so obtained are shown in FIG. 3.

It should be understood that the foregoing disclosure relates to preferred embodiments of the invention and it is intended to cover all changes and modifications of the invention which do not depart from the spirit and scope of the appended claims. 

What is claimed is:
 1. a cathode for use in an electrochemical cell comprising a mixture of lead dioxide (PbO₂) having a water content of between about 0.4 and about 10 percent by weight with a metal fluoride in a mole ratio of between about 3:1 and about 1:3.
 2. The cathode of claim 1 wherein the mole ratio is about 1:1.
 3. The cathode of claim 1 wherein the lead dioxide has a water content of between about 3 and about 6 percent by weight.
 4. The cathode of claim 1 wherein the metal fluoride is selected from the group consisting of cobaltic trifluoride (CoF₃), lead trifluoride (PbF₃), manganese trifluoride (MnF₃), tantalum pentafluoride (TaF₅), niobium pentafluoride (NbF₅), mercuric fluoride (HgF₂), mercurous fluoride (Hg₂ F₂), cupric fluoride (CuF₂), argentous fluoride (AgF), argentic fluoride (AgF₂) and bismuth trifluoride (BiF₃).
 5. A solid electrolyte cell employing the cathode of claim
 1. 6. The solid electrolyte cell of claim 5 employing a lead fluoride-based solid electrolyte.
 7. The solid electrolyte cell of claim 6 wherein a portion of the lead fluoride-based solid electrolyte is incorporated into the cathode.
 8. The solid electrolyte cell of claim 5 employing a solid electrolyte selected from the group consisting of 88PbF₂ -8SrF₂ -4KF, 86PbF₂ -10BaF₂ -4KF, 94PbF₂ -3CaF₂ -3KF, 95PbF₂ -2CaF₂ -3KF, 50PbF₂ -50SnF₂ and 90PbF₂ -5SnF₂ -5KF and an anode selected from the group consisting of lead, lead-coated substrates and lead alloys.
 9. The solid electrolyte cell of claim 8 wherein a lead fluoride-based solid electrolyte is incorporated into the cathode.
 10. The solid electrolyte cell of claim 5 employing a 88PbF₂ -8SrF₂ -4KF solid electrolyte and a lead anode.
 11. The solid electrolyte cell of claim 5 employing a 50PbF₂ -50SnF₂ solid electrolyte and a lead anode.
 12. The solid electrolyte cell of claim 5 employing a 86PbF₂ -10BaF₂ -4KF solid electrolyte and a lead anode.
 13. The solid electrolyte cell of claim 5 employing a 94PbF₂ -3CaF₂ -3KF solid electrolyte and a lead anode. 